The present invention relates to transimpedance amplifiers and in particular to a transimpedance amplifier having a feedback resistive network.
Transimpedance amplifiers convert an input current signal into an output voltage signal. Transimpedance amplifiers may be utilized in a variety of systems and applications including an optical communication system. A transimpedance amplifier typically includes one feedback resistor coupled across an input terminal and output terminal of a voltage amplifier. The current signal applied to the voltage amplifier is passed substantially through the feedback resistor because of the high input impedance of the voltage amplifier. A voltage signal proportional to the input current is thus produced at the output of the voltage amplifier.
To achieve greater gain and sensitivity of the transimpedance amplifier, the resistance of the feedback resistor is typically increased. However, increasing the resistance of the feedback resistor has several drawbacks. First, the feedback resistor has an increased physical size which takes up more space in an environment where there is a premium on such space. Second, resistors manufactured utilizing semiconductor technology processes have a parasitic capacitance associated with them. Therefore, a larger resistor has a larger parasitic capacitance which leads to a reduction in bandwidth of the transimpedance amplifier.
Third, output potential drift of the transimpedance amplifier may be increased in some transimpedance amplifiers. For instance, some transimpedance amplifiers have an input stage including bipolar transistors which have specific tolerances and temperature dependence. In this situation, current through the feedback resistor creates a potential drift in the output voltage of the transimpedance amplifier that is proportional to the feedback resistor value. This drawback is especially prevalent in multistage transimpedance amplifiers with galvanic connection architecture.
Accordingly, there is a need in the art for a transimpedance amplifier having a feedback resistive network with a fixed effective resistance value for maintaining a high effective resistance value while improving on the above drawbacks.
A transimpedance amplifier consistent with the invention includes a voltage amplifier having an input terminal and an output terminal, and a feedback circuit coupled to the input terminal and the output terminal of the voltage amplifier. The feedback circuit includes an impedance element in parallel with a feedback resistive network have a fixed effective resistive value.
An optical communication system consistent with the invention includes a light detector configured to detect an optical signal and produce a current signal representative of the optical signal, and a transimpedance amplifier configured to accept the current signal and provide a voltage signal representative of the current signal. The transimpedance amplifier includes a voltage amplifier having an input terminal and an output terminal, and a feedback circuit coupled to the input terminal and the output terminal of the voltage amplifier. The feedback circuit includes an impedance element in parallel with a feedback resistive network have a fixed effective resistive value.
Another transimpedance amplifier consistent with the invention includes a voltage amplifier having an input terminal and an output terminal, and a feedback resistive network having a fixed effective resistive value. The feedback resistive network is disposed between the input terminal and the output terminal of the voltage amplifier. The fixed effective resistive value is given by the equation: Reff=R1+R2+R1(R2/R3), wherein Reff is the fixed effective resistive value; R1 is a resistance value of a first resistive element; R2 is a resistance value of a second resistive element; and R3 is a resistance value of a third resistive element.
According to yet a further aspect of the invention there is provided a transimpedance amplifier including a voltage amplifier having an input terminal and an output terminal, and a feedback resistive network having a fixed effective resistive value. The feedback resistive network is disposed between the input terminal and the output terminal of the voltage amplifier. The feedback resistive network includes a first resistive element disposed between the input terminal of the voltage amplifier and a node, a second resistive element disposed between the output terminal of the voltage amplifier and the node, and a third resistive element disposed between the node and a ground terminal.
There is also provided a method of increasing the bandwidth of a transimpedance amplifier that includes the steps of: providing a current signal to an input terminal of the transimpedance amplifier; providing an impedance element disposed between the input terminal and an output terminal of the transimpedance amplifier; and setting a fixed effective value for a feedback resistive network having an associated parasitic capacitance, the feedback resistive network disposed between the input terminal and an output terminal of the transimpedance amplifier in parallel with the impedance element, the bandwidth based on the associated parasitic capacitance of the feedback resistive network.